How feasible is it to perform pose estimation on a Raspberry Pi 4 using a Pi-Cam?
If using TFLite python API, you may reach to 1/2 FPS. On downscaled custom Pose architectures you may get more FPS. You can try the TFLite c++ API as well, it might be a bit faster than the python one. 
To reach more than this you would need a TPU or a library that could use GPU computation of the Pi. Nevertheless, I doubt that it would reach 15 FPS with OpenPose open-source library.